1. Field of the Invention
The invention relates to an electrode structure and a semiconductor device, and reduction of on-resistance.
2. Description of the Related Art
In recent years, due to a mobile terminal or the like used widely, a switching element is required to be small and have low on-resistance. Therefore, in a power MOS transistor, for example, operation cells of MOS transistors are integrated in a single semiconductor die so as to be connected parallel, and a high current flows in the vertical direction of the semiconductor die.
For example, in a vertical MOS transistor having a trench structure in which a channel is formed on a side surface of a trench, high density formation of 72 million operation cells per square inch reduces the on-resistance to 12 mΩ.
FIG. 11 shows a conventional semiconductor device, and (a) shows a plan view and (b) shows a cross-sectional view of line x-x.
A semiconductor die 101 has a plurality of operation cells (not shown) on its front surface side, forming a vertical MOS transistor in which a current flows between the front surface and the back surface. In detail, a source electrode 110 and a gate pad electrode 112 are formed on the front surface of the semiconductor die 101. An operation cell has a gate electrode, a gate oxide film and a source region. The source electrode 110 covers all the operation cells and is connected to each of the source regions. Each of the gate electrodes is electrically connected to the gate pad electrode 112. In this structure, the source electrode 110 and the gate pad electrode 112 are electrically connected to leads 116a, 116b through wires 117a, 117b, respectively. A collector electrode 113 is formed on the back surface of the semiconductor substrate 1. The collector electrode 113 is bonded to an island 114 with conductive paste 115 such as solder or the like.
The relevant technique is described in Japanese Patent Application Publication No. 2001-250946.
As described above, the source electrode 110 is formed so as to cover all the plurality of operation cells. However, the wire 117a is bonded to only a part of the source electrode 110, thereby causing differences in distances between a bonding portion 119 of the wire 117a and the operation cells. As a result, the operation cells operate unevenly based on the resistance of the source electrode 110, and the die may be broken due to current concentration.
Therefore, conventionally, many approaches have been taken for minimizing the uneven operation of the operation cells.
For example, as shown in FIG. 12, the source electrode 110 and the lead 116a are connected through a plurality of wires 117a. The wires 117a are bonded to the source electrode 110 in a wide region. This reduces differences in distances between the bonding portions 119 of the wires 117a and the operation cells, thereby providing an even current density. However, a semiconductor device is being miniaturized year after year, and increase of the number of the wires 117a prohibits the miniaturization. Furthermore, the wires 117a need be carefully bonded to the source electrode 110 so as not to short-circuit an interlayer insulation film insulating the gate electrode and the source electrode 110 due to stress caused by the bonding, and the increase of the number of the wires 117a increases the possibility of occurrence of a defect. Furthermore, the cost increases corresponding to the increase of the number of the wires 117a. 
Furthermore, as shown in FIGS. 13A and 13B, the source electrode 110 and a lead 120a are connected through a metal frame 120b formed together with the lead 120a without using a wire. Since the metal frame 120b is bonded to the source electrode 110 in a wide region, each of the operation cells is hardly influenced by the in-plane resistance of the source electrode 110. Furthermore, the metal frame 120b has largely lower resistance than a wire, thereby realizing a semiconductor device having low on-resistance.
However, when the area of the metal frame 120b is increased corresponding to the area of the source electrode 110, conductive paste 122 bonding the source electrode 110 and the metal frame 120b easily becomes uneven and the current density varies accordingly. Furthermore, it is difficult to align the source electrode 110 and the metal frame 120b when these are bonded. Furthermore, the cost increases corresponding to the area of the metal frame 120b. 